1. Field of the Invention
The present invention relates to an apparatus for constant amplitude coded bi-orthogonal modulation and demodulation, which modulates predetermined data to be transmitted into constant amplitude coded bi-orthogonal data, and demodulates the modulated constant amplitude coded bi-orthogonal data into original data and corrects errors in a wireless communication system using the code division multiple access.
2. Description of the Related Art
As for schemes of modulating predetermined data to be transmitted in a communication system, there have been known a direct sequence/code division multiple access (DS/CDMA) scheme, a frequency hopping CDMA scheme, a complementary code keying (CCK) scheme, a multi-code (MC) CDMA scheme, a pulse width (PW) CDMA scheme, a multi phase (MP) CDMA scheme, and the like.
Since the DS/CDMA scheme has a characteristic of constant amplitude and can employ a power amplifier with a simple configuration, its power consumption is relatively low. Further, it can ensure interference rejection capability by substantially performing spread spectrum modulation using Baker codes. However, its data transmission rate is low due to the frequency spread. Moreover, although it provides a variable transmission rate, there is a problem in that its effectiveness is low since the variable range is small. Thus, the DS/CDMA scheme has been mainly employed in IS (Interim Standard)—95.
FH/CDMA-based communication systems have low power consumption and reduced production costs due to the use of simplified power amplifiers, and can ensure interference robustness through the frequency hopping. However, there are problems in that it cannot provide a high transmission rate and it is very limited in the capability of providing a variable transmission rate.
The CCK scheme has been employed in IEEE 802.11b WLAN (Wireless Local Area Network). In the CCK scheme, there are advantages in that its power consumption is low due to the use of signals with a constant amplitude, production costs can be reduced due to the use of simplified power amplifiers, and it can provide a high transmission rate. However, there are problems in that it cannot ensure substantial interference rejection capability or provide a variable transmission rate. In addition, the CCK scheme can be considered as a kind of complex orthogonal modulation. Since code orthogonality for use in enhancing a characteristic of strong resistance against multipath fading may be damaged, there is a problem in that its performance rather degrades under the environment in which delay spread of the multipath fading is low.
The MC/CDMA scheme is a method by which a plurality of orthogonal codes are assigned to a single user. It has been adopted as a standard of the third generation CDMA mobile communication system and described as a standard scheme of 3GPP (Third Generation Partnership Project) or 3GPP2 (Third Generation Partnership Project 2). The MC/CDMA scheme has been developed for providing a user with a high transmission rate and it is more likely to be continually used for a communication system that requires a high transmission rate. However, in a communication system using the MC/CDMA scheme, signals of the MC/CDMA are configured like passband PAM (Pulse Amplitude Modulation) signals when the signals of MD/CDMA are viewed in terms of a time axis. Since there are various amplitudes of transmitted signals so that the system requires a wide range of linear operation in a power amplifier used for a transmitter/receiver, it is difficult to manufacture the power amplifier and production costs thereof increase. If the constant amplitude characteristic can be obtained through a proper coding procedure in signal processing of a base band, passband signals also obtain a constant amplitude so that the production costs of the power amplifier can be greatly reduced and benefit can be obtained in view of power consumption.
As a method of solving these problems incurred due to increase in the levels of modulated signals resulting from the use of multi codes involved with the increased number of channels of transmission data, the PW/CDMA scheme and the MP/CDMA scheme have been suggested.
The PW/CDMA scheme limits output symbol levels of a digital adder to clip portions thereof exceeding a predetermined level value, converts only the values of the remaining levels into pulse widths, and then transmits the pulse widths, so that waveforms of signals always are in a binary form. Although the modulated signals are advantageously in the binary form, there is a disadvantage in that as the number of removed levels in the modulated signals increases, the bandwidth of the modulated signals increases in proportion to the number of the levels.
The MP/CDMA scheme has been suggested as another method of obtaining signals with a constant amplitude. In the MP CDMA scheme, values exceeding a predetermined level value are clipped by a level limiter from symbols with various level values made according to the MC/CDMA scheme, the results obtained through the clipping are converted into phase values, and modulated signals with the converted phase values are amplified by and transmitted from a power amplifier. In other words, the PW/CDMA scheme is a scheme for limiting the output symbols according to the MC/CDMA scheme to a predetermined level and converting the remaining symbols into the pulse widths. The MP/CDMA scheme is a scheme for limiting the level and then converting the remaining symbols into the phases of carrier waves.
The PW/CDMA scheme has an advantage in that the waveform can be simplified, but a problem in that as the number of remaining levels increases, the bandwidth of the modulated signals increase in proportion to the number of remaining levels. However, in the MP/CDMA scheme, the bandwidth becomes constant regardless of the number of levels. Further, in the MP/CDMA scheme, there is an advantage in that the system can be simplified since the level values of the signals are prevented from being increased by clipping portions exceeding a predetermined level value of the signals.
However, the MP/CDMA scheme should use the same number of orthogonal codes as that of transmitted information channels since one orthogonal code is assigned to each channel of data transmitted. Thus, the number of orthogonal codes put to use increases as the number of data channel increases. Accordingly, in the procedure of limiting the levels of the multi-level signals, there is a problem in that the orthogonality of the orthogonal codes is damaged and thus mutual interference between the transmitted signals may happen.